JML,
The capacity related to the amount of fuel that you use is really not important, as stated by Harold the really important thing is to make the tank so that it provides your fuel pump with sufficient inlet conditions such that the pump does not cavitate. This includes making sure that the return line from the return jet (pill) is brought into the tank away from the port that is the pump inlet also this port must be below the normal level of the fuel in the tank. If the tank is very far from the pump make the tank tall so that the fuel level is above the inlet for the pump if possible, also placing the tank in front of the engine will provide some additional pump inlet charge from the acceleration of the car. A basic rule is that the size of the inlet line to the pump cannot be to big. Even if the pump only has a -8 inlet going with a -12 or -16 line will assist the pump inlet conditions. If you make the tank cylindrical and you take the fuel from the bottom, be sure to add a baffle across the tank near the bottom to prevent any vortexing. Lastly don't forget to put a breather in it to allow air to enter as the fuel level drops.
Rex